If you need the exact location,you can have a look at the N900 Service Manual Level 1&2.
I had the same problems, but i changed the housing from black to white. When i was done, the slide function was gone. First i thought i did something wrong, but then i found out that the white housing had no magnet. I took the one from the black and now everything is normal again.
Here are some pics (from the manual) that show where the sensor and the magnet is :


You will see the magnet way before the point where you disasemble the screen. It is just a pic to show you where exactly it is.
Hope this helps some of you.